Directional couplers are used very extensively in technical circuit applications. They are used in phase modifiers, mixers and amplifiers as direction-dependent, de-coupled power dividers. In the field of measurement technology, directional couplers are used for separate measurement of outward and returning waves to and from a device under test (DUT), for example, in the case of network analysers.
In order to realise a coupler with good high-frequency properties in a comparatively compact design with favourable manufacturing costs, directional couplers are preferably manufactured using a monolithically integrated design. Alongside the conventional microstripline technology, directional couplers can also be realised in coplanar technology with a monolithically integrated design. The advantage of a directional coupler in coplanar technology in comparison with a realisation using microstripline technology can be seen, on the one hand, in the attachment of all lines (center conductors, ground conductors) on one side of the substrate. Accordingly, boreholes and through contacts in the substrate, which are necessary for the additional connection of components (e.g. capacitors, inductors) in the microwave circuit are not needed. A further quite substantial advantage of a directional coupler in coplanar technology, by comparison with a realisation in microstripline technology, is that the phase velocity of the even-mode v(e) can be designed to be approximately equal to the phase velocity of the odd-mode v(o), so that the directional loss (directivity) aD can be maximised. This is shown schematically in FIG. 1, the values indicated being provided merely as examples. With a high directivity aD, the power of a wave coupled into the coupling pathway is significantly higher by comparison with the power of a wave coupled to the isolation pathway of a directional coupler. In this context, FIG. 2 presents schematically the basic method of functioning and circuitry for a directional coupler.
EP 0 511 728 B1 discloses a directional coupler in coplanar technology. In the context of directional couplers in coplanar technology realised at the time of publication of this document, the coupling factor was comparatively low, because the distance between the center conductors of an integrated microwave circuit participating in the coupling could not be designed as small as was desirable for reasons of manufacturing technology. Accordingly, the inter-digital coupler (Lange-coupler) presented in EP 511 728 B1 provides a significantly higher coupling factor. This is achieved by means of parallel connection of several center conductors on the substrate. The disadvantage with this coplanar design of the directional coupler is its small band width.
The present invention is therefore based on the object of realising an ultra-broadband directional coupler in coplanar technology as required in measuring technology and particularly in network analysis.
The object of the invention is achieved by a directional coupler in coplanar waveguide technology corresponding to the features of the independent claims 1, 3 and 7.
Advantageous embodiments of the invention are indicated in the dependent claims.